When laser welding is applied at high speed to an object having a gap, part of the laser beam may leak into the gap, resulting in defective welding. To solve this problem, it has been proposed to feed a filler during laser welding, or to combine laser welding with consumable electrode arc welding. The former case requires extra laser energy to melt the filler, and the problem of cost reduction remains unsolved. The latter case has limited application because the rate of deposition of the wire used for arc welding cannot be adjusted independently of the welding current. To solve these problems, the inventors of the present invention have previously proposed a welding method in which a filler is fed to the welding position separately from the wire used for arc welding. As a result, the rate of deposition of the filler can be adjusted independently of the welding current.
FIG. 8 is a schematic diagram showing the conventional method for laser welding proposed by the inventors of the present invention. In this method, laser beam 2 is applied to object 1 to be welded while feeding wire 3 to object 1. Arc 4 is created between wire 3 and object 1. Filler 7 is fed to the welding position of object 1. Laser beam 2 and arc 4 together melt wire 3 into droplets 5, which form molten weld pool 6 above object 1. Molten weld pool 6 solidifies to form bead 8.
FIG. 9 is a schematic diagram showing the principle of the independent adjustment of the rate of deposition of the filler and the welding current in the conventional method proposed by the inventors of the present invention. A melting curve MRA shows the rate of deposition of the wire used for arc welding in a conventional hybrid laser-arc welding method. A melting curve MRF shows the rate of deposition of filler 7 in the conventional method proposed by the inventors of the present invention. In the conventional hybrid laser-arc welding method, when the target rate of deposition to be achieved is VWO, the welding current needs to be I0. In the conventional method proposed by the inventors of the present invention, on the other hand, filler 7 is fed at the rate of deposition VWF shown in the melting curve MRF. Therefore, the rate of deposition of the filler and the wire is as shown in a melting curve MRH, which is the sum of the melting curves MRA and MRF.
In the conventional method proposed by the inventors of the present invention, the target rate of deposition VWO can be reached when the welding current is IH. This means that adjusting the rate of deposition VWF of filler 7 can adjust the welding current IH independently. In other words, in order to achieve the rate of deposition VWO at the welding current IH, the difference between the melting curves MRA and MRH at the rate of deposition VWO and at the welding current IH can be supplemented with the feeding of the filler.
In the actual welding process, the relation between the rate of deposition and the welding current is basically maintained along the melting curve MRH, but various disturbing factors occur. For example, the tip of filler 7 may drop out of molten weld pool 6, and come into contact with the solid portion of object 1 to be welded, or may go into the bottom of molten weld pool 6, making it harder to melt filler 7. To prevent filler 7 from becoming harder to melt, it is necessary to automatically adjust the feeding of filler 7 or even to stop welding. The absence of any function or method to prevent these disturbing factors causes trouble. For example, molten weld pool 6 may be solidified with the tip of filler 7 at the bottom of molten weld pool 6. This may not only cause filler 7 to become non-removable but also damage the filler feeder.
In view of the conventional problems, it is an object of the present invention to provide a method and apparatus for laser welding in which a filler is fed to a welding position, and welding is stopped immediately upon detection of an abnormality in the feeding of the filler.